Variable density window



Patented Nov. 17, 1942 edl UH HWI" VARIABLE DENSITY WINDOW William H. Ryan, Cambridge, Mass., assignor to Polaroid Corporation, Dover, Del., a corporation of Delaware Application November 8, 1940, Serial No. 364,879

4 Claims.

This invention relates to variable density windows.

It is one object of the invention to provide a novel form of variable density window and viewing device wherein it is possible to vary uniformly the intensity of the light transmitted from a predetermined minimum to a predetermined maximum.

Another object of the invention is to provide such a variable density window comprising a plurality of parallel light-polarizing elements of which at least one is movable linearly with respect to the other to vary the amount of light transmitted.

A further object of the invention is to provide such a variable density window wherein each polarizing element is composed of a plurality of parallel strips of polarizing material, adjacent ones of which have their transmission axes relatively perpendicular and each of which is provided, adjacent one surface, with a graded Wave retardation device which is adapted to be combined with a complementary similar device on the other polarizing element to provide a predetermined, variable retardation device between the two polarizers and dependent upon the relative positions thereof with respect to each other.

Other objects and advantages will in part appear and in part be pointed out in the course of the following description of one embodiment of the invention which is given as a non-limiting example in connection with the accompaying drawing, in which:

Figure 1 is a perspective view showing a variable density Window embodying a form of the invention, the view being partly broken away to show means for controlling the movable elements;

Fig. 2 is a detailed sectional view showing more clearly the construction of the window and of the control means therefor; and

Figs. 3-7 are detailed diagrammatic views in horizontal cross-section illustrating the operation of the window shown in Fig. 1, Fig. 3 showing the transmission position of said window, Fig. 7 showing the extinction position, and Figs. 4-6 showing intermediate positions.

Referring to Figs. land 2, element I represents a conventional frame mounted to surround a window aperture in wall I2, which may for example be the inside wall of a railroad vehicle or the like, the outer wall and outer frame being indicated at I4 and I5 respectively. Within frames I0 and I5 is positioned a member I6 provided with a projecting tongue I8 which cooperates with frames I0 and I5 to form a plurality of channels adapted to receive window frames and 22.- As shown in Fig. 2, the window element in trame 20 comprises a central layer 25 between a pairof layers 24 and 2B of glass or other trans- .Cil

between glass or other plastic layers 28 and 32.A

Each of layers and 30 comprises polarizing elements of special polarizing properties combined with wave retardation elements, and their construction and characteristics will be described in more detail hereinafter.

In the illustrated embodiment of the invention, window frame 20 is xedly mounted, but window frame 22 is mounted for sliding movement within its supporting channel in element I6. In accordance with the invention, suitable means are provided for controlling this movement. An example of such means is shown in Figs. 1 and 2 as comprising a lug or nut on frame 22 adapted to receive a screw or bolt 35, which may be mounted in any suitable way within walls I2 and I4. Bolt 35 is in turn controlled by gear 36 meshing with worm 38 journaled in wall I2 and provided with a suitable control knob 40.

It will be seen, therefore, that rotationof knob 40 in either direction will cause bolt 35 to rotate and cause the slidable window element to move linearly with respect to the xed window element. In accordance with the invention, this movement will result in a variation in the amount of light transmitted by the window as a whole, and the reason for this result will be apparent upon reference to Figs. 3-7. It is of course to be understood that the above control means are given only as an illustrative example, and that the invention is in no way to be construed as limited to such a construction.

Referring to Fig. 3, element 25 represents the central layer in the fixed window, and element I0 represents the central layer in the movable window element. Layer 25 is represented as comprising in part a multiplicity of vertical parallel strips of polarizing material of uniform polarization characteristics. 'I'hese strips are preferably of substantially equal width and so arranged that adjacent strips have their transmission axes relatively perpendicular. ample, strips may be considered as having their transmission axes vertical, i. e., perpendicular to the plane of the sheet of the drawing, as is indicated by the short arrows 52, and strips may be considered as having their transmission axes horizontal, i. e., parallel to the plane of the sheet of the drawing, as is indicated by the relatively long arrows 54. Element 35 similarly comprises a multiplicity of polarizing strips 80 having their transmission axes vertical, as indicated by arrows B2, and strips having their transmission axes horizontal, as indicated by arrows 64.

It must be understood that for purposes of illustration Figs. 3-7 are highly exaggerated and diagrammatic. Polarizers having such proper- For exties, however, may be formed in a number of ways, as for example by laminating separate strips of sheet polarizing material to a common backing and arranging adjacent strips with their transmission axes relatively perpendicular. Another suitable method is to orient the molecules of two sheets of suitable plastic material', laminate the two sheets together with their respective directions of molecular orientation relatively perpendicular, and then treat each surface of the resulting sheet in such manner as to render alternate strips thereof light-polarizing.

Suitable materials for use in the latter method are sheets of a transparent linear high polymeric plastic the molecules of which contain hydroxyl groups, and examples include polyvinyl alcohol, polyvinyl acetal and regenerated cellulose. The preferred material is polyvinyl alcohol, and the molecules therein may be oriented satisfactorily by stretching a cast sheet of the material. Two such stretched sheets may then be laminated together with their stretch axes relatively perpendicular and each surface of the resulting sheet may then have alternate strips stained or dyed with a neutral dichroic dye as from a matrix such as a wash-off relief, thev stained strips of each side being separated by unstained strips of equal width and being positioned to coincide with and overlie the unstained strips on lthe other surface.

Suitable neutral dichroic dyes or stains for this process include dichroic cotton dyes and solutions containing tri-iodide ions, and sheets may be prepared by this method of such thinness that their polarizing properties will be substantially the same as those indicated in the drawing, even though adjacent polarizing strips are on opposite surfaces of the sheet.

Bonded to each of the component strips of polarizer in the layer 25, on the surface thereof adjacent layer 30, is a wave retardation element 5'6, and bonded to each of the component strips of polarizer in the layer 30, on the surface thereof adjacent element 25, is a similar wave retardation element 56. 'I'hese retardation elements are preferably positioned with their corresponding optical axes substantially parallel. In the preferred embodiment of the invention, each of strips l56 and 65 is of varying birefringence and so positioned with respect to its optical axes that it will impart to one component of a beam of polarized light transmitted thereto, by the polarizing elements 50 or y55 for example, a relative retardation with respect to the other component varying from one edge to the other edge by a halfwave length. This property is illustrated in the drawing by showing each of the said elements as of wedge shape. It will be seen further that each set of said wedges is so arranged that the low retardation edge of each is adjacent the high retardation edge of the adjacent strip, and that the direction of variation in wedges 56 is opposite to that of wedges 56.

The relative retardation imparted by each of wedges 56 and 55 may vary from approximately zero adjacent one edge to substantially a half wave adjacent the other edge. It will be obvious, however, that essentially the same conditions will exist if the retardation adjacent one edge differs Vfrom that adjacent the other edge by any uneven number of half waves. For example, the relative retardation imparted adjacentv one edge may be a full wave and that imparted adjacent the other may be two and a half waves.

. EIements having properties such as have been 75 tardation plate adjacent the line a'to appr-oiddescribed in connection with the wedges 55 and 66 may be made of any suitable birefringent material, such for example as Cellophane, Kodapak or any similar material, either in wedge shape or by submitting the material to a differential strain which will produce the desired varying birefringence. The birefringent elements 55 and 66 may be positioned in spaced relation to the polarizing elements or may be bonded individually, or as a preassembled sheet, to the polarizing elements with which they are associated. Elements 55 and 56 should be positioned with their corresponding optical axes substantially parallel and at angles of substantially 45 degrees with the transmission axes of the polarizing strips 50, 55, 60 and 65.

As is pointed out above, Fig. 3 shows the transmission position of the two elements comprising the variable density window of this invention. In this position, each pair of coincident polarizing strips have their transmission axes relatively perpendicular. However, each element 56 coincides with a complementary element 5'5 and so imparts an even half-wave retardation to a beam passing from polarizers 50 and 55 to polarizers 55 and 50. Accordingly, light incident on and transmitted by each strip 50, 55 will have its vibration direction rotated through degrees, i. e., twice the angular difference between the transmission axes of the incident polarizing element and the principal optical axes of the superimposed birefringent strips, to a position in which it will be parallel with the transmission axis of the polarizing strips 55, 60 and will therefore be freely transmitted thereby.

The position of maximum extinction is shown in Fig. 7. Here the movable window element 30 is to be understood as having been moved parallel with the fixed window element 25 and laterally with respect thereto in the direction indicated for example by the arrow 45 a distance equal to the width of one of the polarizing strips 50, 65. With the two window elements in this position, light entering for example through the polarizing strips 50 is transmitted as -a vertically vibrating planepolarized beam. The superimposed birefringent elements 55, 55 impart a uniform half-wave retardation to one component of the beam and, since the optical axes of the birefringent elements `are parallel and inclined at an angle of 45 degrees to the transmission axis of the polarizer 50, the direction of vibration of the transmitted polarized beam will be rotated so that it is horizontally vlbrating. In this position it is blocked by the polarizing element 50 which is adapted to transmit only vertically vibrating polarized light. So also, the same will be true of light which is incident on the polarizing strips 55. It will be rotated through an angle of 90 degrees and blocked by the polarizing strips 55.

The intermediate position, that in which the combined elements transmit approximately one half of the light transmitted in the position shown in Fig. 3, is illustrated in Fig. 5. In this ligure the movable window will be understood to have been moved in the direction of arrow 45 a distance equal approximately to one-half the width of each polarizing strip. Accordingly, each polarizing strip -N'overlies approximately one-half of each polarizing strip i0 and one-half of each polarizing strip I5. That portion of the polarizing strip 50 between the dotted lines a and b has associated with it a wave retardation device varying from approximately a one-half wave remately a quarter-wave retardation plate adjacent the line b. That portion of the polarizing strip 50 overlain by the portion of the polarizing strip 50 just described is provided also with a wave retardation plate graded from approximately a one.. quarter wave retardation plate adjacent the line a to approximately a one-half wave retardation plate adjacent the line b. 'I'he sum of the two retardation plates 6 and 65 over the area between the lines a and b is approximately a threequarter Wave retardation. It will be apparent that the sum of the retardation plates 56 and 56 over the area between the lines b and c, i. e., the area including a portion of the polarizing strips 50 and a portion of the polarizing strips 55 will be approximately a one-quarter wave retardation plate. Under the conditions previously explained, the total transmission of the device between the two parallel polarizers 50, '50 separated by the three-quarter wave retardation device will be the same as the transmission between the two crossed polarizers 50, 65 separated by a one-quarter wave retardation device. In each case the light is converted by the retardation elements into circularly polarized light approximately one-half of which is passed by the inner polarizing elements.

Fig. 4 illustrates a position of the element-s midway between that of maximum transmission, as shown in Fig. 3, and that of median transmission, as shown in Fig. 5, and Fig. 6 illustrates a position of the elements midway between that of median transmission, as shown in Fig. 5, and that of maximum extinction, as shown in Fig. 7. In Fig. 4 for example the sum of the birefringence of the elements 5B, 86 between the lines a and b will be approximately seven-eighths Wavelength retardation and the sum of the birefringence of the said elements in the area between the lines b and c will be approximately three-eighths wavelength retardation. In the area between the lines a and b the polarizing elements 50 and 50 have their transmission axes parallel. In the area between lines b and c the polarizing elements 50 and 65 have their transmission axes crossed. Under these circumstances, the light transmitted through the area between the lines a and b will be' substantially equal to the light transmitted through the area between the lines b and c. It will ybe less than the light transmitted bythe device in the position shown in Fig. 3 and greater than the light transmitted by the device inthe position shown in Fig. 5. In the position shown in Fig. 6, where the movable element 30 has been moved in the direction indicated'by the arrow 45 approximately threequarters of the width of the polarizing strip, the birefringence of the two elements 56 and 56 over the area between the lines a and b will be approximately ve-eights wavelength retardation and the birefringence of the said elements 56 and 66 over the area between the lines b and c will be approximately one-eighth wavelength retardation. As the two polarizing elements 50 and 60 are parallel over the area between the lines a and b, and as the two polarizing-elements 50 and 65 are crossed over the area between the lines b and c, the light transmitted through the area between the lines a and bV will be approximately the light transmitted through the area between the lines b and c and will be less than that transmitted by the device shown in Fig. 5 and greater than that transmitted by the device shown in Fig. 7.

nsL

It is thus apparent that in any position of the element 30 with respect to the element 25 the light transmitted by the device is substantially uniform over its entire area. Striations are avoided and the effective density of the entire device may be uniformly and gradually varied from a maximum to a minimum. Variations in the density or in the intensity of transmitted light aresubstantially uniform over the entire area of the window aperture.

It will be apparent also that the total movement of the movable element 30 need be no greater than that of one of the polarizing strips which in turn may be in the neighborhood of one-sixteenth of an inch or even less.l

It will be understood also that while one of the elements has been described as iixed and the other as movable, both-may be movable in directions preferably opposite to one another, in which case each element need move only onehalf the width of a polarizing strip.

Where the birefringent elements are of the order 'of a half-wave retardation device or less, no complications arise by reason of variations in the retardation eiects of the birefringent elements for different wavelengths throughout the visible spectrum. Should it be found that the use of birefringent elements falling within the scope of this invention introduces color to the transmitted light, and this condition may arise if the gradation in birefringence in each element is greater than a one-half wave retardation, then the elements should be selected so as to eiTect the most uniform retardation in approximately the center of the visible spectrum.

The device has been illustrated in Figs. 3 to 7 diagrammatically and has been described as though the birefringent elements were wedgeshaped. It will be understood that wedge-shaped elements need not be employed provided the birefringence is approximately of the character already described. The use of wedge-shaped elements, however, need not render diii'icult the production of laminated elements such as are shown and described for example in connection with Fig. 2. The adhesive employed in connection with the lamination may be of a character to fill the indentations and surface irregularities arising from the use of wedge-shaped birefringent strips. It will be understood further that the device of the invention will function adequately even though the birefringence of the elements employed departs slightly from the preferred, conditions already described. All such modiiications of the device are to be. deemed to fall within the scope of the invention.

Furthermore,l while the device has been described in connection with the use of polarizing strips which have their transmission axes vertical and horizontal, it is to be understood that other forms of devices may be employed. For example, the transmission axes of the polarizing strips 50, 55 and 60, 65 may be perpendicular but at angles of 45 degrees to the horizontal, and this condition may be preferred.

Since certain changes may be made in the above construction and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all .matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having described my invention, what I'claim as new and desire to secure by Letters Patent is:

1. A variable density window comprising, in combination, frame means providing a window aperture, a plurality of sheet-like light-polarizing elements, means for mounting said elements in substantially parallel superimposed relation Within said frame, at least one of said elements being mounted for parallel linear movement with respect to the other, each of s'aid elements comprising a multiplicity of parallel strips of substantially uniform width comprising lightpolarizing material, the transmission axes of adjacent strips being relatively perpendicular, said elements being positioned with the polarizing strips of one substantially parallel to the polarizing strips of the other of said elements, a wave retardation element positioned adjacent that surface of each said strip which is adjacent the other polarizing element, each of said retardation elements imparting to one component of a beam of polarized light incident thereon a relative retardation varying uniformly across the Width thereof by an amount equal to an uneven number of half wavelengths, the optical axes of said birefringent elements being substantially parallel and at angles of substantially 45 degrees to the transmission axes of said polarizing strips, the direction of said variation in retardation in the birefringent elements adjacent one of said polarizing elements being opposite the said direction of variation in retardation in the birefringent elements adjacent the other said polarizing element, and means for moving said movably mounted element.

2. A variable density window comprising, in combination, frame means providing a'wiridow aperture, a plurality of sheet-like light-polarizing elements, means for mounting said elements in substantially parallel superimposed Vrelation Within said frame to overlie said aperture.- at least one of said elements being mounted for parallel linear movement with respect to the other, each of said elements comprising a multiplicity of parallel strips. of substantially uniform width comprising light-polarizing material, the transmission axes of adjacent strips being relatively perpendicular, said elements being positioned with the polarizing strips of one substantially parallel to the polarizing strips of the other of said elements, a. birefringent wave retardation element positioned adjacent that surface of each said strip which is adjacent the other polarizing element, each of said retardation elements imparting to one component of a beam of polarized light incident thereon a relative retardation varying from approximately zero adjacent one edge of said element to substantially one-half wave-length adjacent the other edge thereof, the optical axes of said birefringent elements being substantially parallel and at angles of substantially 45 degrees to the transmission axes of said polarizing strips, the direction of said variation in retardation in the birefringent elements adjacent one of said polarizing elements being opposite the direction of variation in retardation in the birefringent elements adjacent the other said polarizing element, and means for moving said movably mounted element.

3. A variable density window comprising, in combination, frame means providing a window aperture, a plurality of sheet-like light-polarizing elements, means foxmounting said elements in substantially parallel superimposed relation within said frame to overlie said aperture, at least one of said elements being mounted for horizontal parallel linear movement with respect to the other, each of said elements comprising a multiplicity of substantially vertical parallel strips of substantially uniform width comprising light-polarizing material,` the transmission axes of adjacent strips being relatively perpendicular, said elements being positioned with the polarizing strips vof one substantially parallel to the polarizing strips of the other of said elements, a birefringent wave retardation element positioned adjacent that surface of each said strip which is adjacent the other polarizing element, each of said retardation elements imparting to one component of a beam of polarized light incident thereon a relative retardation varying substantially linearly from approximately zero adjacent one edge of said element to substantially onehalf wavelength adjacent the other edge thereof, the optical axes of said birefringent elements being substantially parallel and at angles of substantially 45 degrees tc the transmission axes of said polarizing strips, the direction of said variation in retardation in the birefringent elements adjacent one of said polarizing elements being opposite the direction of variation in retardation in the birefringent elements adjacent the other said polarizing element, and means for moving said movably mounted element.

4. A variable density window comprising, in combination, frame means providing a window aperture, a plurality of sheet-like light-polarizing elements, means for mounting said elements in substantially parallel superimposed relation within said frame to overlie said aperture, at least one of said elements being mounted for parallel linear movement with respect to the other, each of said elements comprising a multiplicity of parallel strips of substantially uniform width comprising light-polarizing material, the transmission axes of adjacent strips being relatively perpendicular, said elements being positioned with the polarizing strips of one substantially parallel to the polarizing strips of the other of said elements, a birefringent wave retardation element positioned adjacent that surface of each said strip which is adjacent the other polarizing element, each of said retardation elements imparting to one component of a beam of polarized light incident thereon a relative retardation varying substantially linearly from approximately zero adjacent one edge of said element to substantially one-half wavelength adjacent the other edge thereof, the optical axes of said birefringent elements being substantially parallel and at angles of substantially 45 degrees to the transmission axes of said polarizing strips, the-direction of said variation in retardation in the birefringent elements adjacent one of said polarizing elements being opposite the direction of variation in retardation in the birefringent elements adjacent ithe other said polarizing element, and means for moving said movably mounted element.

H. RYAN. 

